TCP timeouts

[iliana]

https://tools.ietf.org/html/rfc7766#section-6.2.3

6.2.3.  Idle Timeouts

   To mitigate the risk of unintentional server overload, DNS clients
   MUST take care to minimise the idle time of established DNS-over-TCP
   sessions made to any individual server.  DNS clients SHOULD close the
   TCP connection of an idle session, unless an idle timeout has been
   established using some other signalling mechanism, for example,
   [edns-tcp-keepalive].

   To mitigate the risk of unintentional server overload, it is
   RECOMMENDED that the default server application-level idle period be
   on the order of seconds, but no particular value is specified.  In
   practice, the idle period can vary dynamically, and servers MAY allow
   idle connections to remain open for longer periods as resources
   permit.  A timeout of at least a few seconds is advisable for normal
   operations to support those clients that expect the SOA and AXFR
   request sequence to be made on a single connection as originally
   specified in [RFC1035].  Servers MAY use zero timeouts when they are
   experiencing heavy load or are under attack.